1. Field of the Invention
This invention relates to a method of forming photographic images which comprises intensifying with hydrogen peroxide or compounds capable of releasing hydrogen peroxide. More particularly, this invention relates to an image-forming method which can be used to restrain fog which will occur in the course of intensification, to diminish the influence of intensification inhibitors, and to remarkably increase the intensification effect and maintain the effect in a stable manner.
2. Description of the Prior Art
Many methods of enhancing the image density by development and then intensification of silver halide photosensitive materials are known.
Formation of dyes by the oxidation of a p-phenylenediamine color developing agent due to the decomposition of hydrogen peroxide on the surface of silver catalysts in the presence of a color coupler and subsequent coupling with the color coupler (this phenomenon is referred to herein as "color intensification") has been described in, e.g., Friedman, History of Color Photography, p. 406, 2nd Ed., (1956).
Furthermore, other various photographic methods which take advantage of the decomposition of peroxides on the surface of noble metals have been described in, e.g., German Patent Applications (OLS) Nos. 1,950,102, 1,955,901, 2,044,833, 2,044,993 and 2,120,091 and U.S. Pat. Nos. 3,674,490, 3,684,511, 3,776,730 and 3,765,890.
Further, descriptions of intensification using cobalt complex salts on the surface of noble metals are found in, e.g., Japanese Patent Applications (OPI) Nos. 9,728/73 (corresponding to U.S. Pat. No. 3,834,907), 9,729/73 (corresponding to U.S. Pat. No. 3,765,891), 48,130/73 (corresponding to U.S. Pat. No. 3,822,129), 84,229/74, 84,239/74, 84,240/74 (corresponding to U.S. Pat. No. 3,902,905), 97,614/74 (corresponding to U.S. Pat. No. 3,847,619), 102,340/74 (corresponding to U.S. Pat. No. 3,923,511) and 102,341/74 (corresponding to U.S. Pat. No. 3,856,524).
Moreover, color intensification using halogen acids such as salts of chlorous acid has been described in Japanese Patent Application (OPI) No. 53,826/76 and Japanese Patent Application No. 139,917/74.
Compounds providing an intensifying effect such as the above described peroxides, halogen acids and cobalt (III) complex compounds are referred to an intensifying agents, and processing baths containing these intensifying agents are referred to as intensifying baths.
In color photographic processes with which this invention is concerned, a color photographic material prepared by incorporating cyan, yellow, magenta or other color couplers in silver halide emulsions using various techniques and coating the emulsions on a support is image-wise exposed to light and then subjected to a series of processings to reproduce an image in the photographic material.
The basic processing steps involved in color photographic processing are a color developing step and a silver-removing step. In the color developing step, silver halide containing in the exposed silver halide color photographic material is reduced by a color developing agent to produce a silver image, while the color developing agent is correspondingly oxidized. The oxidized color developing agent reacts with a color coupler to produce a dye image. Then, the color photographic material is subjected to a silver-removing step. In this step, silver which is produced in the prior step, i.e., the color developing step, is oxidized by an oxidizing agent (which is commonly called a "bleaching agent") to silver ion. Thereafter, the photographic material is subjected to a fixing step. In the fixing step, silver ions are dissolved using a complexing agent for silver ions which is commonly called a fixing agent to remove silver from the photographic material. The photographic material which is treated through the above processing steps forms only a dye image. Practical development processing involves the above two basic steps, i.e., the color developing and silver-removing steps, as well as auxiliary steps for maintaining the photographic and physical qualities of the image or improving the storage stability of the image. For example, auxiliary steps include hardening baths for preventing an excess softening of photosensitive layers during processings, stopping baths for effectively stopping the development reaction, image-stabilizing baths for stabilizing the image formed, film-stripping baths for removing a backing layer of the support, etc.
This type of processing of color photographic materials has been usually practiced in the photographic art throughout the world since the 1940's.
The above-described color intensifying methods must provide the same dye density as is obtained when the conventional method is used. Thus, a rapid oxidation of a reducing agent such as a color developing agent must occur on the surface of a small amount of metallic silver catalysts comprising development nuclei in the silver halide grains or developed silver. Therefore, enhancing the activity of the processing bath containing an intensifying agent is desirable, and as to the reducing agents such as color developing agents, those having a higher activity as described in Japanese Patent Application No. (OPI) 11,534/72 (corresponding to U.S. Pat. No. 3,656,950) are preferred to conventional color developing agents.
When the above-described intensifying method with high activity is applied to photographic photosensitive materials containing silver halide as a photosensitive material, color formation occurs also in the unexposed areas in the intensifying bath, resulting in fog (which is referred to as "color intensification fog"). This is an important problem in practical application of intensifying methods.
As one method of restraining fog which is formed during the intensifying step, incorporation of an anti-foggant in the intensifying bath has been attempted. However, the addition of potassium bromide, sodium bromide or the like remarkably inhibits the intensifying action obtained. Also, organic anti-foggants having strong adsorption properties (i.e., compounds containing a mercapto group) remarkably inhibit intensification. On the other hand, compounds having weak adsorption properties (e.g., 6-nitrobenzimidazole) are effective, but if they are added in amounts of more than 1 g/liter, intensification is inhibited to a considerable extent.
Moreover, in using intensifying baths containing peroxides as an intensifying agent, the intensifying effect is remarkably inhibited since potassium bromide which is added as an anti-foggant to a developer and bromide or iodide ions which are released during development of the silver halide enter the intensifying bath.
Further, peroxides, typically hydrogen peroxide, are very unstable in aqueous solutions. In particular, when a developer is mixed with the intensifying bath (e.g., carried over into the intensifying bath), decomposition of hydrogen peroxide occurs in a very short time, resulting in a loss of the image-intensifying action.
Stabilization of an aqueous hydrogen peroxide solution can be achieved by adding a stabilizer such as sodium pyrophosphate or sodium stannate, e.g., as described in, Research Disclosure, 11660; W. C. Schumb, Hydrogen Peroxide, pp. 515-547; etc. However, even the addition of sodium pyrophosphate or sodium stannate is not sufficient to stabilize intensifying baths containing hydrogen peroxide which are contaminated with a developer. The contamination of an intensifying bath with a developer is an essential problem in the image-intensifying method using peroxides. This contamination is inevitable because photosensitive materials are developed and subsequently, without washing or after rinsing for a short time, are treated with an intensifying bath.
Another problem of practical importance is color mixing. Intensifying baths containing peroxides as an intensifying agent cause marked color mixing with the lapse of time after a developer is mixed therein. This color mixing is referred to as color mixing between the layers. Because color formation occurs not only in the layer in which color formation is originally intended but also in another layer (mainly in an adjacent layer) or layers, the purity of the colors is decreased. Therefore, this color mixing is one of the causes for the marked deterioration of image qualities. While not desiring to be bound, color mixing is, it is believed, caused by a reaction of the color developing agent which is mixed into the intensifying bath with the peroxide to produce an oxidized form of the developing agent, the thus produced oxidized form of the developing agent exerting a direct action or an indirect action (the action of a reaction product formed from the oxidized developing agent and other components). Contamination of the intensifying bath with a developer is inherent in image-intensification and, therefore, it is extremely difficult to avoid this problem.
Even when alkylhydroquinone derivatives are incorporated in an intermediate layer, which is a known technique, color mixing due to intensification cannot be prevented.